This invention relates to metal alloys and more particularly to nickel-titanium based alloys.
Nickel-titanium alloys exhibit a shape memory effect at specific temperature depending on their precise composition. This is a well documented fact and was clearly spelled out in U.S. Pat. Nos. 3,558,369 and 3,529,958. To achieve specific transition temperatures ranges (TTR) it is necessary to maintain compositions within .+-.0.025% of a specified value.
It is possible to achieve this narrow compositional range by non-consumable electrode arc melting in water cooled copper hearth furnaces but casting size is limited to ingots of a pound or so. Therefore, this is not feasible for commercial production.
Consumable electrode arc melting has been used commercially to produce 100 pound ingots, but these vary in composition greatly throughout their body, well beyond the requisite .+-.0.025% limitation specified earlier. These variations results from gross segregation as occurs in many castings and also from trace impurities which contaminate the raw materials and which are concentrated by the zone refining effect.
Sintering of nickel and titanium powders followed by extrusion produces a fully dense product but the shape memory response is less than that of the cast and worked ingot, due to less than total homogenization which occurs during reasonable solid state consolidations.
Melting compacted nickel and titanium raw materials in a graphite susceptor is a feasible production method, but as described by Buehler, U.S. Pat. No. 3,529,958, it has certain limitations. These limitations are (1) the requirement to keep the charged melt stock (except for the starter button) free and clear of the graphite sidewalls of the melt crucible and (2) the uncertainty of final composition due to impurities in the charge materials. These seemingly minor effects are capable of producing major shifts away from the intended TTR.
It is to be noted here that chemical analysis of precise titanium content in the presence of approximately 50% titanium is accurate to only .+-.0.025%. Therefore, the use of wet chemical analysis is only reliable for a crude approximation of the TTR.